Here I have just a few suggestions for science books for the kiddos. See this post for the adult version.
Treecology is also a science activity book that people seem to love. Chance are you already have it. Obviously, it focuses on trees, but that does not stop it from being year round, and there are, of course, many non-tree things that relate to trees, and that stuff is covered as well. My review.
Electronics for Kids and The Arduino Project Handbook are great DIY books, the first explicitly for kids, and the second for older kids or adults, or younger kids working with older kids. Click the links to see my reviews.
For kids into math and related fields, check out the Manga guides. Here, I review the latest one on Regression Analysis, and in that post, I’ve got a list of the others.
For smaller kids, there is a new (early last year) David Macaulay book on machines. The book itself is, in fact, a machine.